ANKARA, March 11 (Reuters) - A U.S.-based cleric locked in a feud with the Turkish government has likened Prime Minister Tayyip Erdogan's grip on power to that of the once-dominant military, and warned that political and economic reforms of the past decade are under threat.

In a rare written political commentary, Pennsylvania-based preacher Fethullah Gulen said Erdogan had lost trust at home and abroad because of measures such as curbs on Internet freedom, greater government control of the courts and stronger powers for the intelligence agency.

"A small group within the government's executive branch is holding to ransom the entire country's progress," Gulen, members of whose Hizmet (Service) movement say they number in the millions, wrote in the article published in the Financial Times late on Monday.

"The dominance in politics that was once enjoyed by the military now appears to have been replaced by a hegemony of the executive," he said.

Gulen's support helped cement the rise of Erdogan's Islamist-rooted AK Party over the past decade but a deepening divide between the two former allies spilled into the open in December with an inquiry into alleged government corruption.

The feud poses one of the biggest challenges of Erdogan's 11-year rule. Three government ministers have resigned, and leaked wiretaps of conversations purportedly of Erdogan and those close to him have appeared on social media on an almost daily basis.

Erdogan has slammed the illegal tapping of what should have been encrypted telephone conversations and has described some of the leaked recordings as "fabricated montage".

He portrays the scandal as orchestrated by the cleric through his influence in the police and judiciary, an attempt to smear the ruling party and unseat him ahead of an election cycle starting this month. Gulen denies unleashing the inquiry.

The prime minister has responded by purging thousands of police officers and reassigning hundreds of judges, as well as tightening Internet controls in what his critics see as an authoritarian effort to stem the flow of leaks.

In a veiled reference to Gulen, Erdogan has accused a "parallel state" within the police and judiciary of illegally wiretapping thousands of phones, including his own and those of close aides and government ministers, for several years.

NO RETURN PLANNED

Gulen said Hizmet members, who have cast themselves as the victims of a witch hunt by Erdogan's government, had "no interest in the privileges of power", and that seeking power in the name of religion would contradict the spirit of Islam.

"The Turkish state has long discriminated against citizens and public servants on the basis of their views," he wrote.

"A dark shadow has been cast over achievements of the past decade - the result of insidious profiling of certain groups of Turkish citizens for their views, constant shuffling of civil servants for political convenience, and an unprecedented subjugation of the media, the judiciary and civil society."

Turkish officials have said the reassignments in the police and judiciary and greater government say over the appointment of judges and prosecutors are part of an effort to flush out the influence of Gulen's movement, which they see as an undemocratic, unaccountable force.

Gulen said the government could restore trust only by "renewing its commitment to universal human rights, the rule of law and accountable governance" and said a new constitution drafted by civilians should lie at the heart of this.

Turks have questioned whether Gulen might explicitly come out and suggest his followers vote against Erdogan's AK Party in municipal elections in March or against the prime minister himself, should he stand for president in an August vote.

The cleric ruled out such a move, or any imminent return to Turkey from Pennsylvania, where he lives in self-imposed exile.

"I have never endorsed or opposed a political party or candidate, and will refrain from doing so in future ... I have spent the past 15 years in spiritual retreat and, irrespective of what happens in Turkey, I intend to continue to do so." (Editing by Nick Tattersall and Mohammad Zargham)